Conventionally, to detect leakage of an above-ground or underground storage tank storing liquid such as heavy oil, gasoline, solvent, a leak detector installed in the tank detects fluctuation in a level of the liquid stored in the tank. Based on a result of detection, it is determined whether leakage is occurring (see Japanese Patent Application Laid-Open No. 2000-16500).
FIG. 17 is a schematic sectional view of a tank 100 having a conventional leak detector 110 installed therein. In FIG. 17, the leak detector 110 is passed through a gauging port 102, provided in a top plate 101 of the tank 100, and is installed in the tank 100 so that a flow-rate measuring unit 111 is positioned vertically below a liquid surface LS1 of the stored liquid. The liquid detector 110 detects difference in temperature of the liquid that passes through the flow-rate measuring unit 111 in accompaniment with the fluctuation in a level of a liquid surface LS2 of the liquid stored inside the detector, thereby measuring a flow rate of the liquid. Furthermore, the leak detector 110 detects a state of the tank corresponding to the flow rate based on predetermined leakage criteria, thereby determining whether the leakage is occurring.
To calibrate the leak detector 110, a ventilating path 112a is closed off by sealing or other method to stop the flow of gas between an interior of the tank 100 and an interior of the leak detector 110 and to stop the fluctuation in the level of the liquid inside the leak detector 110. This is done because, when the ventilating path 112a, formed in the cap 112, is in communication with the interior and an exterior of the leak detector 110, the level of the liquid surface LS2 becomes equal to the level of the liquid surface LS1 of the interior of the tank 100. When the fluctuation in the level of the liquid inside the leak detector 110 is stopped, the flow-rate measuring unit 111 detects the difference in temperature of the liquid that stays inside the detector, and a reference value for a liquid-flow-rate computing process is obtained. The leak detector 110 is calibrated based on this reference value.
The present applicant has proposed a leak detector in which a solenoid valve is installed at an upper portion of the leak detector. This solenoid valve is used to close off a small hole, which allows communication of gas between the interior of the detector and the interior of a tank, for a predetermined time to stop the fluctuation in the level of the liquid inside the detector (Japanese Patent Application No. 2002-010148).
However, when a tank that is installed above ground or underground is heated by sunlight or the like directly or through the ground, the top plate or a side plate and the like of the tank undergoes shape deformation due to thermal expansion. In particular, since the liquid surface detecting unit, explained in Japanese Patent Application Laid-Open No. 2000-16500, is fixed to the top plate of the tank, when the top plate undergoes shape deformation, the liquid surface detecting unit is moved along with the top cover. The position of the liquid surface detecting unit with respect to the liquid surface is thus changed greatly. As with the case of heating by sunlight or the like, when the tank that is installed above ground or underground is cooled by rain, snow, or the like, the position of the liquid surface detecting unit with respect to the liquid surface is also changed greatly due to thermal contraction of the tank.
For example, a result shown in FIG. 18 was obtained when the liquid surface detecting unit is fixed to a top plate of tank that is located above ground and that has no leakage of liquid, to detect a liquid-level fluctuation rate of the liquid stored in the tank. In the example shown in FIG. 18, the level fluctuation rate was detected over a period of 12 hours, and during this time, the weather was cloudy and then rainy, and a thunderstorm occurred between the time t1 and the time t2. The level fluctuation rate changed significantly between the time t1 and the time t2 as shown in FIG. 18. This occurred because the tank was cooled by the thunderstorm and underwent shape deformation, thereby causing the liquid surface detecting unit to be moved by the top plate and changing the position of the liquid surface detecting unit with respect to the liquid surface. Thus, with a tank, in which the level fluctuation rate is detected using a liquid surface detecting unit fixed to the top plate, the position of the liquid surface detecting unit with respect to the liquid surface often varies with a variation of the environmental temperature. Consequently, it is difficult to detect leakage with high precision based on the fluctuation in the level of the liquid. Erroneous leakage detection thus makes early detection of leakage of a tank difficult and leads to environmental pollution due to leaked liquid.
Meanwhile, when the ventilating path 112a is closed off to calibrate the conventional leak detector 110, the gas inside the leak detector 110 becomes sealed in the space surrounded by the liquid surface LS2 and inner walls of the leak detector 110. In this state, if the tank 100, in which the leak detector 110 is installed, is heated by sunlight or the like, the temperature of the interior of the tank 100 rises and the temperature of the interior of the leak detector 110 also rises. As a result, the gas existing in the interior of the leak detector 110 undergoes thermal expansion and increases in volume. The gas inside the leak detector 110 thus pushes down the liquid surface LS2 by the pressure increase and causes the liquid to flow by a minute amount with respect to the flow-rate measuring unit 111. Appropriate calibration of the leak detector is thus made difficult often and the precision of leak detection of the tank 100 is thereby degraded. This causes erroneous leak detection, which makes early detection of leakage of a tank difficult and leads to environmental pollution due to leaked liquid.
When the tank 100, with which the ventilating path 112a of the installed leakage detector 110 is closed, is cooled by rain, snow, or the like, the temperature of the interior of the tank 100 decreases and the gas existing in the respective internal portions of the tank 100 and the leak detector 110 contracts. Thus, inside the leak detector 110, the internal pressure decreases, the liquid surface LS2 becomes raised, and a minute amount of the liquid flows with respect to the flow-rate measuring unit 111. Appropriate calibration of the leak detector 110 is thus made difficult often to cause the same problems as a case in which the pressure of the interior of the leak detector 110 increases.
With the leak detector proposed by the present applicant, the solenoid valve that is installed at the upper portion closes off the small hole, which allows the flow of gas between the interior of the detector and the interior of the tank, to seal off the air inside the detector. Therefore, the same problems due to pressure fluctuations inside the detector can occur with this arrangement as well.
The present invention has been achieved in view of the above circumstances, and it is an object of the present invention to provide a leak detector and a leak detecting system using the same, with which the degradation of the leak detection precision due to variation of the environmental temperature is restrained to enable early detection of liquid leakage in high precision.